1. Field of the Invention
The present invention refers to a driving or movement system for moving the injection assembly up to the mould closing assembly on injection moulding machines for plastics.
2. Description of Prior Art
According to the prior art, in the injection moulding procedure, use is made of injection moulding machines or apparatus wherein resins consisting of plastic materials are heated in order to be brought to a molten state in a heating barrel and are injected at high pressure into a cavity of a mould so as to fill it. The resin in the molten state is cooled inside the mould so as to harden and form the moulded product. At this point the mould is opened and the moulded product, which can be taken to storage or to subsequent processing stages, is ejected.
The injection moulding apparatus comprises an injection assembly and a moulding assembly. The injection assembly comprises a barrel externally heated by means of electric resistances and containing a plasticizing screw.
The plasticizing screw has two functions. In the first function the plasticizing screw, by rotating, conveys and plasticizes the material in the front part of the barrel. In the second function the plasticizing screw, by translating axially, transfers the material, by now in the molten state, from the barrel to the inside of the mould.
In particular the mould is divided into two mould halves mounted respectively on a fixed plate and on a movable plate. In this manner the mould can be opened and closed by moving the movable plate back and forth.
The moulding assembly comprises a double toggle mechanism that provides the movable plate with the movements necessary for opening/closing of the mould as quickly as possible. Furthermore the double toggle mechanism develops the mould closing force to be able to resist the pressure of the material during injection.
The two injection and moulding assemblies are disposed one in front of the other on a machine base. In particular, the injection assembly is not permanently constrained to said base. In fact it can slide on shoes or columns to translate back and forth to allow the injection nozzle to be moved away from or toward the mould.
By means of a driving apparatus the injection-plasticization assembly is given the motion of translation and a contact pressure between the injection nozzle and the mould is generated. Said contact pressure between nozzle and mould is necessary during injection of the plastic material to prevent the plastic material from escaping in the area of contact between the nozzle and the mould.
FIG. 1 shows schematically and partially a moulding machine according to the prior art comprising an injection and plasticization assembly 1 and a moulding assembly 30, shown partially. The machine has a movement apparatus for the injection assembly, according to the prior art, designated as a whole by reference numeral 100.
The moulding assembly 30 comprises a movable mould half, not shown, and a fixed mould half 8 mounted on a fixed plate 6 and in turn mounted integrally on a base 5 of the machine.
The injection assembly 1 comprises a hopper 2 into which solid plastic material is fed and a plasticizing barrel 3 inside which a plasticizing screw (not shown in FIG. 1) operates. The plasticizing barrel 3 ends in an injection nozzle 7 for injection of molten material into the mould.
The plasticizing assembly 1 is mounted slidably on profiled guides 4 provided on the base 5 of the moulding apparatus. In this manner the plasticizing assembly 1 can translate toward the moulding assembly 30. That is to say, the injection assembly 1 moves toward the fixed plate 6 until the injection nozzle 7 comes into contact with the fixed mould half 8.
Movement of the injection assembly 1 is achieved by means of the driving apparatus 100. The driving apparatus 100 has a transmission 109 with metal cables or ropes 109, 110 respectively connected to the front and rear part of the injection assembly 1.
The cables 109 and 110 extend respectively on pulleys 113 and 114 which are mounted on the fixed plate 6 of the mould and on the machine base 5, respectively. The cables 109 and 110 are driven by a common winding drum 112, in turn powered by an electric motor 111.
The phase of bringing the injection assembly 1 toward the moulding assembly 30 is obtained by winding the cables 109 on the drum 112 while the cables 110 are unwound. Vice versa for withdrawal of the injection assembly 1 from the moulding assembly 30 the direction of rotation of the electric motor 111 and thus of the drum 112 which causes winding of the cables 110 and unwinding of the cables 109 is inverted. Clearly the cables 109 and 110 can be parts of the same cable wound for a few turns on the winding drum 112.
The cables 109, in particular, are connected to the structure of the injection assembly 1 by means of small pistons 116 that act on a series of disc springs 115 provided in special cylindrical seats 131 of the injection assembly structure. The disc springs 115 when compressed make it possible to achieve the contact force necessary to hold the injection nozzle 7 against the fixed mould half 8 during injection of the plastic material. In fact, during this injection phase, the high pressure on the plastic material would lead to retraction of the injection assembly 1.
The movement apparatus 100 according to the prior art has various drawbacks due to slipping of the cables 109 and 110 under load and the problems relating to winding of the cables 109 and 110 on the pulleys 113 and 114 or on the small-diameter drums 112.
In fact, because of the high forces that have to be developed to maintain the contact between the injection nozzle and the mould, large-diameter cables must be provided, whereas because of problems of size the diameter of the idler pulleys and the winding drum cannot be increased too much.
In order to solve the problems of slipping between the cables and the pulleys, an alternative prior art solution provides for the cables 109 and 110 to be replaced with respective chains, maintaining the same structure of the movement system.
Both solutions, that providing for use of cables and that providing for use of chains, require adjustment of the movement system, so as not to create a backlash between the cable or chain and the pulley during reversal.
Moreover, both cables and chains must be long enough to be able to connect the front part and the rear part of the injection assembly, resulting in greater difficulty in driving or moving them.
The object of the invention is to eliminate the drawbacks of the prior art, providing an apparatus for moving the injection assembly that is able to ensure a high level of performance, allowing a high movement speed and a tight seal between the injection nozzle and the mould.
Another object of the present invention is to provide such a driving apparatus of the injection assembly that is reliable and able to reduce problems of jamming and adjustment of its mechanical components to a minimum.
Yet another object of the present invention is to provide such a driving apparatus for the injection assembly that is practical, economical, versatile and simple to make.
The movement apparatus for moving the injection assembly of a moulding machine, according to the invention, has an injection assembly slidably mounted on a machine base and driven or moved by means of a drive or movement unit that drives a metal chain transmission, in which one end of the chains is connected to the injection assembly. The other end of the chains is connected by means of spring means to a fixed frame of the machine.
The chains, driven by the drive unit, generate the forward movement of the injection assembly toward the moulding assembly, whilst the spring means keep the pull of the chains constant. The backward movement of the injection assembly away from the moulding assembly is generated by return means. Said return means are preferably extension spring means which connect the injection assembly to the frame of the machine.
However, the return means can also be another chain transmission driven by the same drive unit as the first chain transmission or by another drive unit.
The advantages of the movement apparatus of the injection assembly according to the invention are obvious. In fact, said movement apparatus allows the use of metal chains which ensure a greater reliability with respect to the metal cables used in the apparatus of the prior art.
Furthermore, the return means make it possible to avoid using long chains connected to the rear part of the injection assembly to allow its return.
Moreover, the spring means connected to one end of the chain make it possible always to keep the chain under tension, thus avoiding the creation of backlash between the chain and its driving sprocket wheels and therefore also the problems of jamming and adjustment of the transmission sprocket wheels of the chain.